We know little about the role that male sex steroids play in cerebral ischemic injury, even though male sex is an acknowledged risk factor for clincal stroke. Adult male animals sustain larger ischemic damage for a comparable insult relative to age-matched females, suggesting a male "ischemia-sensitive" phenotype. Our ongoing work with testosterone, the most abundant circulating androgen, indicates that the steroid can shape ischemic cell death. Castration decreases, and testosterone repletion increases, cell death after middle cerebral artery occlusion in the sexually mature, adult male rat or mouse. However, testosterone naturally declines with age (the andropause), and this event appears to change completely the role that androgens play in experimental stroke. In middle age, testosterone replacement to plasma values of the young animal provides neuroprotection from focal cerebral ischemia. This project evaluates how testosterone acts in the aging male brain to improve ischemic outcome and to determine if the androgen's actions are sex-specific. Our central hypothesis is that in middle-aged injury, testosterone is aromatized to estradiol (E2) and signals through neuroprotective estrogenic pathways. In contrast, testosterone is shuttled in young male adults into a dihydrotestosterone (DHT) signaling pathway with deleterious effects on tissue outcome. In aim 1, we will determine if pre-vs post-ischemic testosterone treatment in middle-aged male rats alters ischemic cell damage and if this is androgen receptor mediated. Next, we will determine if testosterone's actions in aging brain are dependent on its rapid aromatization by the cP450 aromatase (Aim 2) and subsequent signaling through estrogen receptor independent pathways (Aim 3). Aims 4 and 5 center on testosterone's mechanisms in the young male, determining if DHT formed from testosterone acts in an age-specific manner to promote ischemic cell death (Aim 4) and to suppress endogenous bc!2 gene expression after focal cerebral ischemia (Aim 5). These aims will yield new information about testosterone as it shifts from an enhancer of damage to a neuroprotectant over the life cycle of the male.